Analysis of neural mechanisms involved in normal brain function and disease could be dramatically accelerated using novel measurement methods that report neural processing events with molecular specificity, noninvasively, and across the entire brain. Here we propose to develop a molecular sensor platform for imaging the two most important amino acid neurotransmitters, glutamate and gamma-aminobutyric acid (GABA), by noninvasive magnetic resonance imaging (MRI). Sensors will be formed by modifying a family of amino acid binding proteins with gadolinium chelating groups. The resulting probes will enable neurochemical imaging with behaviorally-relevant temporal resolution, on the scale of entire brain regions or intact brains. This qualitatively new capabilit will enable us and others to answer specific questions about how the dominant excitatory and inhibitory neurotransmitters in the vertebrate central nervous system participate in multiple facets of brain physiology. Our sensor design is based on the large ligand-dependent structural changes induced in so-called venus flytrap domains (VFDs). By conjugating gadolinium-containing groups to VFDs at strategically chosen amino acid positions, we expect to generate MRI contrast agents with sensitivity to the ligands that naturally bind to each corresponding VFD. In Aim 1, we will use this strategy to generate glutamate-sensitive MRI contrast agents based on VFDs from the bacterial periplasmic binding protein YbeJ. In preliminary work on this Aim, we have already observed large MRI changes due to glutamate binding to gadolinium-derivatized YbeJ variants, indicating the overall promise of our approach. In Aim 2, we will apply our glutamate sensors in rat brains and use them to map glutamate release patterns during a representative somatosensory stimulus. We will also compare glutamate and conventional functional MRI (fMRI) activation maps to examine the hypothesis that hemodynamic fMRI measures closely reflect glutamatergic signaling. Our recent detection of dopamine release using a less potent form of MRI sensor in rat brains represents a precedent for the proposed in vivo work, and again indicates promise. In Aim 3, we will extend our glutamate sensor design strategy to target GABA, the dominant inhibitory neurotransmitter in the brain. The proposed research has